Generally speaking, a liquid crystal display device or an organic EL (Electro Luminescence) display device of an active matrix type includes: a substrate on which a thin film transistor (Thin Film Transistor; hereinafter also referred to as “TFT”) is formed as a switching element for each pixel (hereinafter also referred to as “TFT substrate”); a counter substrate on which a counter electrode, color filters, and the like are formed; and an optical modulation layer, e.g., a liquid crystal layer, provided between the TFT substrate and the counter substrate.
On the TFT substrate, a plurality of source lines, a plurality of gate lines, and a plurality of TFTs respectively disposed at intersections therebetween, pixel electrodes for applying a voltage across the optical modulation layer such as a liquid crystal layer, storage capacitor lines and storage capacitor electrodes, and the like are formed.
The construction of a TFT substrate is disclosed in Patent Document 1, for example. Hereinafter, with reference to the drawings, the construction of a TFT substrate disclosed in Patent Document 1 will be described.
FIG. 12(a) is a schematic plan view showing the TFT substrate in outline, and the FIG. 12(b) is an enlarged plan view showing one pixel of the TFT substrate. FIG. 13 is a cross-sectional view of a TFT and terminal portions of the TFT substrate shown in FIG. 12.
As shown in FIG. 12(a), the TFT substrate includes a plurality of gate lines 2016 and a plurality of source lines 2017. Each region 2021 surrounded by these lines 2016 and 2017 defines a “pixel”. In a region 2040 of the TFT substrate other than the region (displaying region) where the pixels are formed, a plurality of connecting portions 2041 for allowing the plurality of gate lines 2016 and source lines 2017 to be respectively connected to a driving circuit are provided. Each connecting portion 2041 constitutes a terminal portion for providing connection to external wiring.
As shown in FIG. 12(b) and FIG. 13, a pixel electrode 2020 is provided so as to cover each region 2021 defining a pixel. Moreover, a TFT is formed in each region 2021. The TFT includes a gate electrode G, gate insulating films 2025 and 2026 covering the gate electrode G, a semiconductor layer 2019 disposed on the gate insulating film 2026, and a source electrode S and a drain electrode D respectively connected to both end portions of the semiconductor layer 2019. The TFT is covered by a protection film 2028. An interlayer insulating film 2029 is formed between the protection film 2028 and the pixel electrode 2020. The source electrode S of the TFT is connected to a source line 2017, whereas the gate electrode G is connected to a gate line 2016. The drain electrode D is connected to the pixel electrode 2020 within a contact hole 2030.
Moreover, a storage capacitor line 2018 is formed in parallel to the gate line 2016. The storage capacitor line 2018 is connected to a storage capacitor. Herein, the storage capacitor is composed of a storage capacitor electrode 2018b which is made of the same conductive film as the drain electrode D, a storage capacitor electrode 2018a which is made of the same conductive film as the gate line 2016, and the gate insulating film 2026 interposed therebetween.
On the connecting portion 2041 extending from each gate line 2016 or source line 2017, the gate insulating films 2025 and 2026 and the protection film 2028 are not formed, but a connection line 2044 is formed so as to be in contact with an upper face of the connecting portion 2041. As a result, electrical connection between the connecting portion 2041 and the connection line 2044 is ensured.
As shown in FIG. 13, in the liquid crystal display device, the TFT substrate is disposed so as to oppose a substrate 2014 on which a counter electrode and color filters are formed, with a liquid crystal layer 2015 interposed therebetween.
In the recent years, it has been proposed to form a channel layer of a TFT by using an oxide semiconductor such as IGZO (InGaZnOx), instead of a silicon semiconductor film. Such a TFT is referred to as an “oxide semiconductor TFT”. An oxide semiconductor has higher mobility than does amorphous silicon. Therefore, an oxide semiconductor TFT is able to operate more rapidly than an amorphous silicon TFT. Moreover, an oxide semiconductor film is formed through simpler processes than those of a polycrystalline silicon film, and therefore is also applicable to devices which require a large area.
Patent Document 2 describes a TFT substrate having oxide semiconductor TFTs. In FIG. 4 and the like of this document, an oxide semiconductor TFT construction for use as a protection circuit of a display device is described.